The invention relates to an electrolysis stack for an electrolyzer, said electrolysis stack having a plurality of electrolysis cells. It further relates to an electrolyzer, and to an energy supply system comprising such an electrolyzer and a renewable energy source that is connected to the electrolyzer.
An electrolyzer is known from http://de.wikipedia.org/wiki/Elektrolyzeur (available on Jul. 15, 2013) and is a device which brings about a conversion of materials with the aid of an electric current (electrolysis). Given the wide variety of different types of electrolysis, there also exists a multiplicity of electrolyzers, such as e.g. an electrolyzer for hydrogen electrolysis.
In the case of such an electrolyzer for hydrogen electrolysis, known embodiments of which include alkaline electrolyzers or acid or PEM electrolyzers, water is broken down into hydrogen and oxygen.
Current thinking (http://de.wikipedia.org/wiki/EE-Gas, available on Jul. 15, 2013) relates to the use of surplus energy from renewable energy sources, as in times of above average solar power or wind power generation in corresponding installations, to drive electrolyzers for the purpose of producing so-called EE gas.
In this case, a (hydrogen electrolysis) electrolyzer which is attached to the renewable energy source first generates hydrogen using the energy from the renewable energy source, and said hydrogen is then used in a Sabatier process with carbon dioxide to produce methane. The methane can then be fed into an existing natural gas network, for example, and therefore allows energy to be stored and transported to the consumer and can therefore reduce the load on an electricity network. Alternatively, the hydrogen which is generated by the (hydrogen electrolysis) electrolyzer can also be reused directly, e.g. for a fuel cell.
An electrolyzer usually consists of a plurality of electrolysis cells which are electrically connected in series and combined to form an electrolysis stack, or respectively to form a plurality of electrolysis stacks which are interconnected in series or in parallel. The number and type of electrolysis cells and their interconnection are determined by a specific (U-I) characteristic curve of the electrolyzer in this case.
In this case, an electrolysis cell in turn consists of an anode and a cathode. In the case of an alkaline electrolysis cell, an electrolyte, usually potassium hydroxide solution, and a separator are situated between the anode and the cathode. In the case of a PEM electrolysis cell, a gastight polymer electrolyte membrane is situated between the anode and the cathode.
The electrolyzer requires a DC voltage for operation.
If energy is supplied to an electrolyzer via an AC voltage network or a three-phase voltage network, use of a converter is therefore necessary. Using this converter, the DC voltage level can be controlled and a working point of the electrolyzer can then be adjusted according to its characteristic curve.
If electrical energy is supplied to an electrolyzer by a renewable energy source, i.e. the electrolyzer and the renewable energy source are connected together via a shared DC voltage rail in this case, it can also be operated without the use of a converter as a control means.
For example, an electrolyzer can be attached directly via the shared DC voltage rail to the direct current supply of a photovoltaic field (PV field), i.e. to PV generators composed of interconnected PV modules, without any need for a connection to an electricity network via a photovoltaic inverse rectifier in this case.
If an electrolyzer is connected via the shared DC voltage rail to the renewable energy source/DC voltage source, e.g. the PV field (a direct current generator of a wind energy installation, referred to simply as wind energy installation below, is also possible here), a shared working point of renewable energy source and electrolyzer is established on the shared DC voltage rail.
This shared working point of electrolyzer and renewable energy source/PV field/wind energy installation (both together also referred to simply as energy supply system below) is produced as illustrated in FIG. 1 as an intersection point of a (specific) I-U characteristic curve of the renewable energy source/PV field (continuous curve; current I and voltage U, also referred to simply as characteristic curve below) and the I-U characteristic curve of the electrolyzer (dashed curve; current I and voltage U).
In this case, the characteristic curve of the renewable energy source, e.g. the PV field or the wind energy installation, is not fixed, but is influenced by (environmental) parameters. For example, the characteristic curve of a PV field depends on strength of Insolation on the PV field, environmental temperature at the PV field, PV cell surface with which the PV field operates, and aging of the PV field.
Corresponding environmental parameters which influence the characteristic curve are also known for other renewable energy sources, e.g. wind strength in the case of the wind energy installation.
It is also shown in FIG. 1 that the characteristic curve of the (in this case) PV field has a point, a so-called MPP (maximum power point), at which the PV field or the renewable energy source delivers a maximum power yield (maximum power). The MPP also moves with changing environmental parameters.
In order to achieve maximum possible power yield, the renewable energy source is preferably operated in the region of its MPP (which changes in response to changing environmental parameters). Simply expressed, working point and MPP should correspond whenever possible.
If the renewable energy source is connected to the electrolyzer via the DC voltage rail, and the shared working point of renewable energy source and electrolyzer is therefore established via the DC voltage rail as a shared intersection point of both characteristic curves, it is also desirable here for the shared working point to lie in the region of the (changing) MPP of the renewable energy source, and therefore in the region of the maximum power of the energy source.
It is known that corresponding agreement or matching of the shared working point for optimum operation of renewable energy source and electrolyzer (or of the energy supply system) can be achieved by means of a DC-DC regulator. However, this has the disadvantage that such a DC-DC regulator is cost-intensive and is or may be susceptible to error.